When spinner-type gas volume meters are installed in delivery stations, downstream from expanders or obstacles such as valves, bends, double-bends, constrictions, tees, or other non-rectilinear duct elements, meter accuracy runs the risk of being affected by the disturbances such obstacles generate in the flow. One of the solutions that has been adopted until now has been to leave a long straight length of pipe between the meter and the last obstacle situated upstream therefrom. Not only is that solution detrimental to making stations compact, it also turns out that long straights are not always sufficient for sufficiently attenuating the effects of certain major disturbances.
Thus, in order to eliminate disturbances while also reducing the size of stations, it is often necessary to make use of flow conditioners. Such equipment modifies the structure of a flow, causing its characteristics to approach those of an undisturbed flow of the kind obtained downstream from very long straight lengths of pipe, known as a "fully developed" or an "established" flow.
Numerous types of flow conditioner are in existence, some of which are mentioned in the ISO 5167 standard or in the AGA Report 7 standard. Nevertheless, although existing conditioners are capable of reducing the rotation of a gas stream, known as "swirl", very few of them reduce in satisfactory manner the asymmetry of the flow or the violent jet effects that are observed downstream from certain expander-regulators. They are designed for use with upstream and downstream straights of length corresponding to several times the nominal diameter of the pipes, which limits the advantage thereof when it is desired to make expander-and-meter stations that are compact.
More particularly, patent document GB-A-2 235 064 describes a flow conditioner comprising a perforated plate having a central hole and a set of other holes distributed in concentric rings centered on the central hole. In each ring, the holes are regularly distributed and they all have the same diameter so that the device is axially symmetrical. The diameter of the holes decreases on going outwards from the central hole towards each of the concentric rings. Such a flow conditioner is quite effective, providing it is used in combination with long straight lengths of pipe, but it turns out to be insufficient for installation in a compact expander-and-meter station.
Patent document U.S. Pat. Nos. 5,495,872 and 5,529,093 describe a flow conditioner comprising a perforated plate that does not have a central hole, but that has a set of holes which are distributed essentially in two to four concentric rings centered on the center of the perforated plate. Within any one ring, the holes are regularly spaced apart and have the same diameter. The diameter of the holes varies from one ring to another. A set of three small-diameter holes at the vertices of an equilateral triangle is disposed in the center of two concentric rings to mitigate the absence of a central hole. Such a perforated plate likewise does not enable performance to be optimized and the above-mentioned documents insist on using the perforated plate in combination with an upstream device for countering gas stream swirl, the device being made up of a set of tubes and being spaced apart from the perforated plate, pipe straights of non-negligible length must also be included in the flow conditioner upstream and downstream from the perforated plate.